This invention relates to a method of controlling progressive freeze-concentration. The so-called freeze-concentration method is coming to be noted for concentrating target liquids of foods and medicaments for obtaining concentrates of a superior quality. Although both the suspension crystallization method and the progressive freeze-concentration method are known as examples of the freeze-concentration method, the present invention relates to the progressive freeze-concentration method and in particular to a control method for the progressive freeze-concentration of a target liquid while a target liquid and a cooling medium are being circulated.
As disclosed, for example, in Shokuhin-To-Gijutsu (“Foods and Technologies”), Vol. 318, pp. 1–8 (December, 1997) and the Proceedings of Japan Society of Food Engineering ((2000), page 85), it has been known as a method of progressive freeze-concentration to freeze-concentrate a target liquid by causing a cooling medium to circulate through a cooling-medium flow route of a freeze-concentration apparatus while causing the target liquid to circulate through a target-liquid flow route of this apparatus so as to gradually form ice crystals on wall surfaces of the target-liquid flow route.
In such a method of progressive freeze-concentration, the flow rate of the target liquid and the growth speed of the ice front at the ice-liquid interface during the freeze-concentration are two important factors that significantly influence the effective freeze-concentration of the target liquid. If the flow rate of the target liquid at the ice-liquid interface is too slow, the content ratio of solute to the ice crystals becomes accordingly too high. If it is too fast, on the other hand, the load on the freeze-concentration apparatus becomes accordingly large. If the growth sneed of the ice crystal is too fast, the content ratio of solute to the ice crystal becomes accordingly too high. If it is too slow, on the other hand, the time of freeze-concentration becomes accordingly too long. In such a progressive freeze-concentration method as described above, it is important to adjust both the flow rate of the target liquid and the growth speed of the ice front at the ice-liquid interface during the freeze-concentration, but there has not been any method proposed for controlling these factors. Ordinarily, either the rotary speed of the pump for circulating the target liquid and the temperature of the cooling medium are kept at fixed levels, or the flow rate of the target liquid and the growth speed of ice front at the ice-liquid interface during the freeze-concentration are estimated and the rotary speed of the pump for circulating the target liquid is gradually lowered or the temperature of the cooling medium is lowered as the time for freeze-concentration elapses. With such prior art methods, however, it is not possible to optimize the flow rate of the target liquid or the growth speed of the ice front at the ice-liquid interface during the freeze-concentration, allowing these factors to undergo significant changes.